The interplay between electron correlation and spin–orbit coupling in solids has been proven
to be an abundant gold mine for emergent topological phases. Here we report the results of
systematic magnetotransport study on bandwidth-controlled pyrochlore iridates R2Ir2O7 near
quantum metal-insulator transition (MIT). The application of a magnetic field along [001]
crystallographic direction (H//[001]) significantly decreases resistivity while producing a
unique Hall response, which indicates the emergence of the novel semi-metallic state in the
course of the magnetic transformation from all-in all-out (AIAO, 4/0) to 2-in 2-out (2/2)
spin configuration. For H//[111] that favours 3-in 1-out (3/1) configuration, by contrast, the
resistivity exhibits saturation at a relatively high value typical of a semimetal. The observed
properties can be identified to reflect the emergence of multiple Weyl semimetal states with
varying numbers of Weyl points and line nodes in respective spin configurations. With tuning
effective bandwidth, all these states appear to concentrate around the quantum MIT region,
which may open a promising venue for topological phenomena and functions. (c) The Author(s) 2017